Emulsions form as a result of many forms of processing and are used extensively by the food, cosmetic and coating industries.
Recent advances in the generation of emulsion droplets have led to further applications in drug delivery and oil extraction. The ability to generate single and multiple emulsions with controlled morphology has allowed for their use in fabricating a variety of functional materials, including microgels and liposomes, which are used for drug delivery; polymersomes used for encapsulating and protecting sensitive molecules, such as drugs, enzymes, other proteins and peptides, and DNA and RNA fragments; and colloidosomes, which are hollow elastic shells whose permeability and elasticity can be precisely controlled. The generality and robustness of colloidosomes makes them a potential candidate for cellular immunoisolation.
Double emulsions can provide advantages over simple emulsions (e.g. oil-in-water, water-in-oil or water-in-water) for encapsulation, such as the ability to carry both polar and non-polar cargos, and improved control over the release of therapeutic molecules. The preparation of double emulsions typically requires mixtures of surfactants for stability and the formation of double nanoemulsions, where both inner and outer droplets are less than 100 nm. However, double nanoemulsions have not yet been achieved in the prior art.
The fluids used in the prior art for the generation of a simple water-in-water emulsion, are combinations of dextran and polyethylene glycol (PEG), or PEG and sodium citrate, or PEG and potassium phosphate, which are characterized by an interfacial tension on the order of 0.1 mN/m. To generate a simple emulsion, the currently used techniques employ a method based on a Polydimethylsiloxane (PDMS) device with a piezoelectric actuator. The piezoelectric actuator is set in a flexible microchannel to bring in the inner fluid (the encapsulated fluid) and a given voltage amplitude together with a well-chosen frequency are used to control the formation of droplets in the outer fluid. An example of this prior art approach can be found in I. Ziemecka, V. van Steijn, G. J. M. Koper, M. Rosso, A. M. Brizard, J. H. van Esch, and M. T. Kreutzer, Lab Chip, 11, 620 (2011), herein incorporated by reference. Additionally, the prior art is directed to a method based on multi-level rounded channels which lead to the generation of simple water-in-water emulsions as described in D. Lai, J. P. Frampton, H. Sriram, and S. Takayama, Lab on a Chip, 20, 3551 (2011) herein incorporated by reference. Furthermore, the prior art has described the application of ultralow surface tension two-phases flow to generate droplets of controlled and uniform diameter with a good production rate. The introduction of a perturbation through a mechanical vibrator has been suggested to produce droplets in air as in P. Haas, ind. Eng. Chem. Res., 31, 959-967 (1992) and Ref: I. Ziemecka, V. van Steijn, G. J. M. Koper, M. T. Kreutzer, and J. H. van Esch, Soft Matter, 7, 9878 (2011) both of which are herein incorporated by reference.
Currently, most of these emulsions contain organic compounds as one of the phases, which compounds are often costly, toxic, flammable and harmful to the environment. The risks of having these compounds in the final structures have led to difficulties in getting them approved for biological applications and internal consumption. Therefore, it is highly desirable to replace the organic solvents with aqueous-based two-phase systems (ATPS) solvents. Nowadays ATPS solvents are widely used for the separation and purification of proteins, antibodies, DNA, cells, cell organelles and even nanoparticles. An all-aqueous emulsion can be generated using ATPS, which forms two immiscible aqueous phases with attractive features, such as their biocompatibility or non-toxicity.
Despite the numerous advantages of ATPS, the use of droplet microfluidic techniques to generate water in water (W/W) emulsions has been limited. One major reason is the low interfacial tension, which prevents the formation of droplets by classical methods. Furthermore, it is a challenge to produce ATPS emulsions in double emulsion form because of the extremely low interfacial tension, even when using microfluidic capillary techniques. Most of the emulsions are made up of two immiscible fluids with an interfacial tension typically of tens of micro Newtons per meter. When the interfacial tension becomes extremely low, typically 100-1000 times lower than the interfacial tension of typical oil-water interfaces, the generation of the emulsion is prevented. The low interfacial tension leads to a reduction in the driving force for liquid jets to break up into droplets by Rayleigh-Plateau instability.
Therefore, what is needed is a method and apparatus that applies perturbation techniques (e.g. mechanical, electrical, etc.) with the formation of simple and double ATPS emulsions where the interfacial tension of both aqueous phases is low.